The invention relates in general to the field of microlithography.
The invention relates in this field to a method for correcting a field-constant astigmatism of a projection objective of a microlithography projection exposure apparatus.
The invention also relates to a projection objective of a microlithography projection exposure apparatus.
The invention also relates to a method for the microlithographic production of micropatterned components and to a micropatterned component produced in such a way.
Micropatterned components, for example highly integrated electric circuits, are produced microlithographically in the semiconductor industry.
For this purpose, at least one layer made from a light-sensitive material, for example a photoresist, is applied to a substrate, for example a silicon wafer. The substrate thus coated is subsequently exposed in a projection exposure apparatus. During exposure, an object, a reticle, provided with an appropriate patterning is imaged with the aid of a projection objective onto the light-sensitive layer. After the development of the photosensitive layer, the substrate is subjected to an etching or deposition process, as a result of which the uppermost layer is patterned on the reticle in accordance with the pattern. The still remaining part of the light-sensitive layer is then removed. This process is repeated until all the layers are applied to the substrate.
Because of the steadily rising demands being made on the integration density of micropatterned components, it is necessary to apply to the substrate microfeatures whose dimensions are becoming ever smaller. Since the resolution is proportional to the wavelength of the projection light, the tendency is toward smaller wavelengths of the exposure light. At present, use is already being made of light in the deep ultraviolet (DUV) and even in the extreme ultraviolet (EUV) spectral region, the latter spectral region lying at about 13 nm. Since there are no materials that are still sufficiently transparent in such a spectral region, projection objectives for the EUV region are constructed from reflective optical elements. Such a reflective microlithography projection objective is described in document DE 100 37 870 A1. The projection objective described there has six mirrors as optical elements.
With the rising demands placed on the resolution of the microfeatures to be imaged, aberrations caused by the projection objective are increasingly posing a problem.
Astigmatism is an aberration the at least partial correction of which is the subject matter of the present invention.
In the case of astigmatism, an object point lying on the optical axis is not imaged in a punctiform fashion by the projection objective. If a beam emanating from an off-axis object point and impinging obliquely on an optically imaging element is decomposed into such rays as lie in the so-called meridian plane that is defined by the optical axis and the principal ray of the beam, and into such rays as lie in the so-called sagittal plane, in which the principal ray likewise lies but which runs perpendicular to the meridian plane, it is observed that the meridian rays have a different focus than the sagittal rays. The difference between the two foci of the meridian rays and the sagittal rays is denoted as the astigmatic difference. In the case of an object pattern that is formed, for example, from two mutually perpendicular lines, one line is sharply imaged in a specific image plane by an astigmatic element, while the other line is unsharp or blurred in this image plane, and the relationships are precisely the other way round in another image plane spaced apart from the first image plane. Thus, there is no image plane in which the two lines can simultaneously be sharply imaged and, in the case of a projection objective this leads to a loss of imaging accuracy because of the complexity of the features to be imaged.
In the case of so-called field-constant astigmatism, which is also denoted as on-axis-astigmatism, the aberration caused by astigmatism is constant when seen over the image field, that is to say it is not dependent on the spatial coordinate of the image field. The cause of astigmatism can lie, for example, in fabrication errors of the surfaces or of the material (for example an inhomogeneous refractive index for objectives with refractive elements) of the optical elements, in deformations of optical elements owing to external forces or torques that are caused by nonideal mounts or screw joints and their assembly, in non-uniform thermal expansions of near-pupil optical elements during operation (for example owing to radiation with light) and/or in a nonuniform degradation of the material of the optical elements during use of the projection objective. In the case of objectives with refractive elements, the refractive index can change owing to radiation with light, for example.
A field-constant astigmatism is observed, inter alia, in the case of lithography optics of rotationally symmetrical design. Lithography optics of rotationally symmetrical design are those which exhibit rotational symmetry with reference to at least one optical axis, and image an object onto a circular full image field centered with reference to the optical axis.
If a projection objective includes rotationally symmetrical optical elements whose surface has a deformation with twofold symmetry, an attempt can be made during the adjustment to rotate one or more optical elements by large angles about the optical axis, which is also denoted as clocking. This is described, for example, in the article by David M. Williamson entitled “Compensator Selection in the Tolerancing of a Microlithographic Lens”, in SPIE “Recent Trends in Optical Systems Designs II”, vol. 1049, 1989, pages 178-186.
Field-constant astigmatism occurs, however, not only with rotationally symmetrical projection objectives, but also in the case of projection objectives that image an object or a part of an object onto only a partial annular image field that is off-axis, that is to say lies outside the optical axis. Such projection objectives are denoted as not rotationally symmetrical. Such a projection objective is the projection objective in accordance with document DE 100 37 870 A1 already mentioned above.
Such not rotationally symmetrical projection objectives typically have mirror symmetry with reference to a plane of symmetry that is defined by at least one optical axis and the center of the image field.
Unlike the rotationally symmetrical projection objectives, because of the lack of rotational symmetry with such a projection objective the field-constant astigmatism cannot be corrected by clocking.
Moreover, with rotationally symmetrical projection objectives an attempt may be made to deform at least one surface of an optical element in order to correct the astigmatism if clocking has not resulted in the success desired. However, since the field-constant astigmatism can vary with time during operation of the projection objective, such a deformation requires an actively deformable optics, and this is associated with a substantial level of structural complexity, and thus with a high outlay on cost.
Since clocking does not generally constitute a solution to the correction of astigmatism in the case of not rotationally symmetrical projection objectives, the only possibility left would be to provide an actively deformable optics in general, with the disadvantage of the very high technical requirements with reference to accuracy of setting, operational stability and high costs.